Fuel efficiency DMU compared to loco-hauled

[Western 52]

As fuel prices are so high these days, fuel must be a significant part of the overall cost of running a diesel train. There are many trains worked by various DMU formations, including many that cover long distances. Some TOCs, such as TPE, TfW and Chiltern make use of locomotive push/pull formations though.

I can imagine that where a long DMU formation is used – say a 10 car Voyager – that may well use more fuel than a 10 coach loco hauled train? I know there are many variables to consider such as the nature of the route and the stopping pattern of the train. For a shorter train, a DMU is probably more efficient, but some of the loco formations used are actually quite short formations.

My question is where does the balance lie in terms of fuel efficiency?

 

[Bletchleyite]

I seem to recall it being quoted in the past as about 5 coaches being the tipping point.

 

[eldomtom2]

Surely it also depends on the ratio of power cars to trailers?

 

[tbtc]

Bear in mind that the “fuel efficiency” needs to be seen against other changed costs, e.g. a heavy locomotive at the front of a train does a lot more damage to track than the distributed weight of a DMU (more damage if you’re wanting to top and tail)

As you say there are a number of other variables (can platforms accommodate one or two locomotives without fouling junctions/ signals… If you’re not top and tailing then are there sufficient run round facilities… what about services where there’s currently portion working out diagrams where units are only doubled up for certain journeys ) … just focusing on the fuel efficiency might give you the answer that you want (more loco hauled, yay!) but it’s only variable and overall costs might be a lot more complicated

Also, the services were DMUs were replaced by loco hauled generally had a lot more to do with stock shortages than because loco haulage is superior in its own right (e.g. TOCs have had to bring in loco hauled stock midway through franchises on routes like the Fife Circle, Cumbrian Coast, Chiltern gaining capacity into Marylebone… but even stock designed to be longer than a five coach DMU - like the EMR bid based on maxing out trains for the 240m platforms at St Pancras - saw an order for 810s capable of being doubled up rather than loco hauled… The only exceptions being First’s decision to replace some 185s with Mk5s… which looks increasingly daft… and GWR/ ScotRail’s adventures with HSTs which haven’t been plain sailing either)

 

[Bald Rick]

Very simply, A modern loco hauling a train with a gross weight of, say, 300t, will use roughly the same amount of fuel as a modern DMU hauling a train with a gross weight of 300t, f acceleration rates and top speed are the roughly the same. it’s physics after all.

 

[Bletchleyite]

Very simply, A modern loco hauling a train with a gross weight of, say, 300t, will use roughly the same amount of fuel as a modern DMU hauling a train with a gross weight of 300t, f acceleration rates and top speed are the roughly the same. it’s physics after all.

It was more about a larger number of smaller engines traditionally being less efficient than one big one. Maybe that doesn't hold true any more though?

Plus for LH you're dragging another vehicle around.

 

[hexagon789]

ScotRail's HST sets consume approximately 2.1 times the fuel as a 3-car 170 for a given mileage. That does not take into account the fact that their better acceleration allows for more time spent coasting though.

 

[Richard Scott]

I know someone who used to work for Cross Country and he said there was little in it for fuel consumption between a 5 car 221 and a 7 car HST.
There's more to it than just total mass of train. 5 smaller engines are likely to be more inefficient than one or two larger ones. Having said that mass is a factor and modern DMUs are very heavy, I believe 185 and 221 cars are over 50 tons each so a 5 car 221 is over 250tons. What's a mk 5? I wouldn’t be surprised if a 5 car mk 5 rake plus 68 is actually lighter than a 221 set or a 6 car 185?
As quoted above rule of thumb used to be 5 cars or more then loco hauled was more fuel efficient.
I know when Sprinters first came out they were quoted at 6.6mpg per car. Expect a 68 betters 1mpg so balances at around 6 coaches with that comparison but I'd imagine a 185 is very thirsty and each car would be nowhere near 6.6mpg?

 

[hexagon789]

What's a mk 5? I wouldn’t be surprised if a 5 car mk 5 rake plus 68 is actually lighter than a 221 set or a 6 car 185?

A Nova 3 comes in at 245.8 tonnes tare including the 85t Cl. 68.

A three-car 185 unit is 163 tonnes tare.

A five-car 221 is 280.7 or 283.2 tonnes.

A four-car 220 is 194.6 tonnes.

 

[Ken H]

I think there was a plan to build some 3 car cl158 type trains with 2 x 600hp engines rather tha 3 x 400hp ones. So one car would be a trailer. Dont think it got far. But they reckoned savings in capital cost and maintenance. Dunno about fuel consumption. Think squeezing a 600hp engine under the floor was a challenge.

 

[sw1ller]

I think there was a plan to build some 3 car cl158 type trains with 2 x 600hp engines rather tha 3 x 400hp ones. So one car would be a trailer. Dont think it got far. But they reckoned savings in capital cost and maintenance. Dunno about fuel consumption. Think squeezing a 600hp engine under the floor was a challenge.

You’d never get the power down. Some of our 158’s struggle to pull away when it’s a bit moist now and that’s just 350hp

 

[61653 HTAFC]

I remember the old rule of thumb being 5 cars of a DMU being the tipping point where loco-hauled made more sense (assuming all vehicles powered, which has been the case for all DMUs from the mid-1980s to the present day, barring the Stadlers). At the time, DMUs did offer much better acceleration which tipped the scales in their favour, but the 68s narrowed that gap significantly.

 

[HammerDown154]

A Nova 3 comes in at 245.8 tonnes tare including the 85t Cl. 68.

A three-car 185 unit is 163 tonnes tare.

A five-car 221 is 280.7 or 283.2 tonnes.


Silly question but is the fuel load/weight factored in. Thanks

A four-car 220 is 194.6 tonnes.

A Nova 3 comes in at 245.8 tonnes tare including the 85t Cl. 68.

A three-car 185 unit is 163 tonnes tare.

A five-car 221 is 280.7 or 283.2 tonnes.

A four-car 220 is 194.6 tonnes.

Sorry to ask a silly question but is the fuel load/weight factored in and does that make a significant difference to the overall efficiency? Thanks

 

[hexagon789]

A Nova 3 comes in at 245.8 tonnes tare including the 85t Cl. 68.

A three-car 185 unit is 163 tonnes tare.

A five-car 221 is 280.7 or 283.2 tonnes.


Silly question but is the fuel load/weight factored in. Thanks

A four-car 220 is 194.6 tonnes.


Sorry to ask a silly question but is the fuel load/weight factored in and does that make a significant difference to the overall efficiency? Thanks

All tare weights - so empty of fuel and passengers.

 

[HammerDown154]

All tare weights - so empty of fuel and passengers.

Thanks

 

[AlastairFraser]

You also have to account for auxiliary power provided to coaching - some older diesel locos are inefficient at providing auxiliary power, to an extent.

 

[D7666]

Very simply, A modern loco hauling a train with a gross weight of, say, 300t, will use roughly the same amount of fuel as a modern DMU hauling a train with a gross weight of 300t, f acceleration rates and top speed are the roughly the same. it’s physics after all.

Spot on.

The specific fuel consumption - gram of fuel per kW hour - of say Cummins QSK19s distributed under floors of 185 / 180 / 22x is more or less the same as, say, a QSK50 (which will be in class 99) QSK78 of 3300 hp if that were used in a Type 5.

So same gross load, same accn and dccn rate, same cruise speed, very similar.

The ETH load or 'hotel' load as some like to say is also very similar - the same amount of energy is needed to heat and light 10 coaches of LH stock as 10 coaches of underfloor DMU stock if the stock build is similar.

Where things differ is where things go astray. Compare a 2580 hp 12LDA28s 47/8 with 5xQSK19 => 5 x 750 hp = 3750 hp in a 5car 22x. That is chalk and cheese, different in every parameter you can think of, even before you compare weight. It was interesting some of the publicity blurb that was put out when 22x were new about how more emissions friendly they are - ISTR one said something like 50% less emissions. Well on specific fuel data (gram fuel burnt per kW hour) maybe, but that conveniently forgets 3750 horses c.f. 2580 horses means there are 1.45 more horses each churning out 50% less emssions, which significantly reduces the argument.

 

[edwin_m]

DMUs also have the advantage that some engines can be shut down when moderate levels of power are needed, so that the working ones are close to their most efficient output. There are of course some practical difficulties here, such as the efficiency and reliability impacts if engines are started more often from cold.

 

[Ken H]

DMUs also have the advantage that some engines can be shut down when moderate levels of power are needed, so that the working ones are close to their most efficient output. There are of course some practical difficulties here, such as the efficiency and reliability impacts if engines are started more often from cold.

Trans Pennine did that. But the big issue is more cycles of cooling and heating leading to stresses and cracking.

 

[Snow1964]

There is also maintenance costs, it obviously costs more (both in parts and time) to maintain 5 engines than one big engine.

There is also a downtime and capital cost element to it, if the power car is a separate loco, can have extras and maintain and fuel it whilst the hauled passenger stock is in service. This might sound trivial but when you consider a service like Cardiff-Portsmouth is rarely 5car because awaiting maintenance and parts then counts a lot against the DMU is cheaper argument.

But ultimately fuel consumption is thermal efficiency of the engine and the mass to be accelerated. At higher speeds aerodynamics also become bigger proportion. Basic physics, fuel energy used being converted into kinetic (momentum) energy.

 

[Richard Scott]

You also have to account for auxiliary power provided to coaching - some older diesel locos are inefficient at providing auxiliary power, to an extent.

Can you explain how they are inefficient?

But ultimately fuel consumption is thermal efficiency of the engine and the mass to be accelerated. At higher speeds aerodynamics also become bigger proportion. Basic physics, fuel energy used being converted into momentum energy.

Take it you mean chemical to kinetic energy?

 

[Western 52]

Do modern DMUs use waste heat from the engines to heat carriages? If they do, I'd guess that would tip the fuel efficiency balance towards them to a small extent. Not much use for air conditioning though, which will still need an electrical supply.

 

[hexagon789]

Do modern DMUs use waste heat from the engines to heat carriages? If they do, I'd guess that would tip the fuel efficiency balance towards them to a small extent. Not much use for air conditioning though, which will still need an electrical supply.

150-156 Sprinters do; the newer trains tend to have air-con, so it's maybe not necessary on those.

 

[AlastairFraser]

Can you explain how they are inefficient?

Sure, my understanding is that old diesel electrics have to produce power for their DC motors, but the auxillary power systems like e.g. lighting use AC power, so you need a rectifier.
Electric tech has moved forward a lot in the last 50 years, so the older rectifiers lost quite a chunk of power converting from DC to AC.

 

[Bletchleyite]

Sure, my understanding is that old diesel electrics have to produce power for their DC motors, but the auxillary power systems like e.g. lighting use AC power, so you need a rectifier.
Electric tech has moved forward a lot in the last 50 years, so the older rectifiers lost quite a chunk of power converting from DC to AC.

If a separate alternator was more efficient (as a mechanical DMU has to have) surely you'd just do that, making it the same?

 

[Fen Enthusiast]

I was wondering how some DEMUs and BMUs either with power packs or one or so vehicles having diesel generators like the BMU Flirts, Flexes and DEMU Thumpers and the short lived class 210s compare to DMUs, other DEMUs, BMUs and diesel hauled trains?

 

[AlastairFraser]

If a separate alternator was more efficient (as a mechanical DMU has to have) surely you'd just do that, making it the same?

You would, but we're talking about the older locos here, which didn't provide for that AFAIK.

 

[Krokodil]

A Nova 3 comes in at 245.8 tonnes tare including the 85t Cl. 68.

A five-car 221 is 280.7 or 283.2 tonnes.

The Nova 3 has 291 seats (261 Std, 30 1st). The 221 has 250 seats (224 Std, 26 1st). Per seat therefore I'd expect running costs to be lower per passenger on the Nova 3. The 68 is also 15 years younger so should be more efficient to begin with.

 

[Dan G]

IIRC large diesel efficiency hasn't improved since the 1990s due to tighter emissions regulations (which tend to work against efficiency)

 

[edwin_m]

Sure, my understanding is that old diesel electrics have to produce power for their DC motors, but the auxillary power systems like e.g. lighting use AC power, so you need a rectifier.
Electric tech has moved forward a lot in the last 50 years, so the older rectifiers lost quite a chunk of power converting from DC to AC.

A rectifier converts AC into DC, not the other way round. The Electric Train Heating standard for BR coaching stock allows for the supply down the train to be DC or AC at quite a wide range of frequencies, but this requires each coach to generate its own AC for whatever might need it. Traditionally this was done by a motor-alternator set, which may be what you are thinking of.

But a DMU would have a similar issue. The engine would drive a generator producing DC or an alternator producing AC, but the frequency of the latter would vary with engine speed. So it would still need a M-A set or in more recent designs electronics to produce whatever voltages and frequencies are needed for auxiliaries.